1. Field of Invention
The present invention relates to an improved rheological additive for organic and other systems which is in a pourable, liquid form at ambient temperatures and which need not contain a solvent to achieve its liquid state. Such an additive provides improved viscosity control and a variety of other rheological properties to many types of organic and water-based systems without adverse environmental effects. The present invention also relates to a thickened composition or system containing the new rheological control additive.
2. Description of the Prior Art
The art has constantly sought materials, generally referred to as thickeners, thixotropes, rheological additives or rheological control agents, which are used to control the rheological properties of various liquid compositions including inks, epoxies, paints, coatings, polyesters and greases. It has been generally recognized that to be effective, especially for coating compositions, such rheological control agents must provide proper viscosity and rheological characteristics to the systems in which they are used. In this regard, organophilic clays have shown to be useful to thicken various organic and solvent-based compositions. Exemplary U.S. Patents which describe various kinds of organically-modified clays are U.S. Pat. Nos. 4,208,218; 4,410,364; 4,412,018; and 4,517,112. Fumed silica and precipitated silica have also been used to thicken certain types of organic compositions.
There are, however, drawbacks with the use of organically modified clays and silicaceous materials for thickening organic compositions. Since both organically modified clays and fumed silica exist in solid particulate form, these materials generally must be added during the grind stage of manufacture of the compositions to be thickened. Additionally, dusting problems are associated with the use of such organically-modified clay and silicaceous products during manufacturing procedures. In addition, the use of these types of additives can lead to a loss of gloss in the cured coating. Fumed silica in particular is dusty and difficult to handle because of its low bulk density. Furthermore, these additives exhibit extremely rapid recovery following shear, thereby limiting the leveling or smoothness of the applied composition. Organoclays are sometimes sold as gels, where the organoclay is dispersed in an organic liquid, in order to avoid the need to use a solid thickener.
It has long been known in the art to use various materials as thickening compositions to thicken aqueous systems. Aqueous systems include both water-based and latex-based paints, coatings, inks, construction materials, cosmetics, and wood stains utilized in various aspects of a civilized industrial society. Depending on the composition of the aqueous system, the products made with these thickeners can be useful as decorative and protective coatings, paper coatings, cosmetics and personal care items, adhesives and sealants, inks, petroleum drilling fluids, joint compounds, and the like.
Many aqueous thickeners are known, including natural, modified-natural, and synthetic. Natural thickeners, for instance, include casein and alginates. Modified-natural thickeners include modified cellulosic, including methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose. These products vary in their thickening efficiency and flow and leveling properties. Synthetic thickeners in recent years have assumed increased importance, particularly in the viscosity control of aqueous and latex paints and coatings. The synthetic thickener serves several roles in aqueous systems. In latex and aqueous paints and coatings, for instance, the thickener provides improved stability and pigment suspension, and improved application properties.
Synthetic rheological thickeners include various acrylic polymers and maleic anhydride copolymers. Two patents issued to Rheox Inc., a subsidiary of NL Industries, Inc., describe a family of polyurethane thickening compositions. These patents, U.S. Pat. Nos. 4,499,233 and 5,023,309, describe these synthetic thickeners as the polyurethane reaction product of polyisocyanates, polyether polyols, and modifying agents which contain at least two active hydrogen moieties, terminated by a capping agent. One type of synthetic thickener used extensively in commercial applications is a low molecular weight polyurethane characterized by hydrophobic groups interconnected by hydrophilic polyether groups, and is disclosed in Rohm & Haas U.S. Pat. Nos. 4,079,028 and 4,155,892.
One similar type of synthetic thickener is a water-soluble, thermoplastic organic polymer having a number of monovalent hydrophobic groups incorporated in the internal portion of the polymer molecule. U.S. Pat. Nos. 4,496,708 and 4,426,485, issued to Union Carbide Corporation, describe such thickeners as water-soluble comb polymers.
The above-described rheological additives, useful in aqueous systems, have been often referred to by the term "associative" thickeners. Associative thickeners are so called because the mechanism by which they thicken may involve associations between species in the thickener molecules and other surfaces, either on other molecules of the thickener, or on molecules in the system to be thickened.
Some of the problems of use, dispersibility and reduction of gloss associated with the solid particulate type of thickeners such as silica are overcome with the use of polyamide rheological additives. For example, U.S. Pat. No. 4,778,843 describes a solid polyamide rheological additive especially designed for organic solvent-based compositions, which comprises the reaction product of polycarboxylic acid, an active hydrogen compound of a specified carbon chain length and structure, and a monocarboxylic acid capping agent, wherein the additive exhibits excellent efficiency and dispersibility, and is effective when predispersed in an organic solvent. Recently-allowed U.S. Pat. No. 5,349,011 describes a polyamide-ester rheological additive, especially for organic solvent-based compositions, which comprises the reaction product of polycarboxylic acid, an active hydrogen composition of a specified structure, an alkoxylated polyol, and a monocarboxylic acid capping agent. Said additive exhibits excellent efficiency and ease of dispersibility for aliphatic solvent-based coating compositions, and is effective when dispersed into a solvent.
U.S. Pat. No. 5,034,444 describes an anti-sag additive for non-aqueous coating compositions which is the reaction product of an alkoxylated aliphatic nitrogen-containing compound, an aliphatic diamine or mixtures thereof, and an organic polycarboxylic anhydride or acid, an alkanediol polyepoxide ether or mixtures thereof. The additive provides excellent anti-sag and storage stability properties, particularly for high solids coating compositions, without causing a significant increase in viscosity.
U.S. Pat. No. 4,072,641 describes polyamide resins useful as flexographic ink vehicles which are prepared by reacting polymeric fatty acids, an alkylene diamine, a mono amino alcohol which is neither branched nor ethoxylated, and a chain-stopping agent which agent, includes a particular branched chain monocarboxylic fatty acid, which resin is liquid at room temperature. The resultant polyamide resin is resistant to gelling during storage.
Levels of rheological thickening additives varying between 0.1% and about 10%, based on the total weight of the system to be thickened, have been found to be useful. The aforesaid associative thickeners are in most cases highly efficient viscosity improvers, even though they have low to middle range molecular weight. They are not sensitive to degradation. They are versatile, in that not only do they thicken virtually unlimited types of systems, but they also impart many beneficial auxiliary properties. In certain paints and coatings, especially, they not only thicken, but in certain cases also provide superior flow and leveling properties, as well as excellent viscosity control under both low and high shear conditions.